Flanged dish end 1

[flamby]

The flange of a dish end for a pressure vessel application is coming out very high. I am interested in knowing the pros and cons of using a thinner flange with stiffeners all around. Is this acceptable in practice? What are real dangers of doing this as far I can show the structure is reasonably safe as far as structural calculations are concerned?

TIA

 

[codeeng]

Could you elaborate. Are you talking about a body flange or the straight flange portion of the head? What do you mean about "very high"?

 

[flamby]

Its a hemispherical dish with a flange like [Þ] all around for bolting it to the parent vessel. The thickness comes out to be higher that 250mm which is not available as a plate in the market.

 

[prex]

As you seem to be talking of a relatively high pressure vessel, I don't think that using rib stiffeners is a viable solution: some may say that nothing is forbidden under ASME VIII, you only have to prove your design, but this one is certainly not a solution that is provided as is by any pressure vessel code.
The thickness of a flange is strongly related to the distance between the ID and the bolting circle: if this distance is unusually high in your design, you could try solutions for reducing it (e.g. more bolts with a smaller diameter).
If you need more help, you should give more details on your design conditions and geometry.

prex

http://www.xcalcs.com

Online tools for structural design

 

[flamby]

prex,

Thankyou for your comments. Here is a link to what I am up to.

http://server3.uploadit.org/files/flamby-screen.jpg

Any more of your comments shall be helpful.

 

[unclesyd]

As stated above come back with more details, design conditions and material of construction.

Has the assembly been fabricated?

I have never seen a flange design as you have in the drawing.

There are several details that are very troublesome, the bolts and arraignment, the gasket, the gussets, and the weld detail where flange attaches to the head among others.

 

[bvi]

If you are pressed to make a thinner flange, and in this case I doubt it is required, you can use a flat face flange with metal to metal contact outside of the bolt circle, per Section VIII, Div 1, Appendix Y. Those generally use o-ring type gaskets and the requied flanged thickness will be significantly less than per Appendix 2.

 

[prex]

Confirming the points by unclesyd, this is certainly not an acceptable design for a pressure vessel (what are the design conditions?), and would hardly pass any kind of design review: personally would never sign off such a drawing as an engineer.
Main points of concern have been recalled above: to these I would add:
-depending on gasket type (not represented in the drawing) flange thickness might be not sufficient purely under a stiffness point of view
-the flange will certainly require a machining after welding those huge gussets
-use of screws is unusual for such geometries: studs are preferred for fatigue resistance reasons (but the rest of this design is absolutely poor from a fatigue standpoint)
-bolts can be screwed from one side only: this is not really a major concern but is not a good solution
-nuts will certainly not fit between the gussets, due to gusset welds
-....?
Just another comment, it it helps: if your main concern is flange thickness, and you don't need the space inside the head, you could consider a cover with a partial spherical dome. You'll get a thicker head but, by suitably choosing the radius of the spherical dome, also a thinner flange.
Consider also that, till you get by analysis very solid arguments, there is no reason to think that the cover flange, as it is now, would be differente from the (existing?) shell flange (unless there has been a major change in design conditions).
Please flamby, come back with something more realistic or hire a vessel engineer.


prex

http://www.xcalcs.com

Online tools for structural design

 

[JStephen]

One thing I note is that the gussets look too short to do much. That is, they would give large radial loads into the head, which would probably overstress the head locally.

Can you maybe make this into two or three parallel vessels, each of a smaller diameter? That would help considerably.

Have you sized the bolts yet? Seems with the required thickness shown, it would be difficult to fit enough bolts in there.

 

[aybee]

agree with all the other guys have said, have you considered a single piece forging for the flange with cylindrical shell sections for attachement to head and to shell.that way you can get good shell/head to flange section welds , lower stresses at the flange to shell attachment..Aybee

 

[flamby]

Thankyou all for confirming my doubts that such a design is absolute crap.

 

[robsalv]

Flamby

You've set my engineering spider senses tingling!!!! Are you located in a regulatory PV regime? I can't see that design ever flying unless it's for your own back yard project.

In theory your proposal could work, but the practical problems are very highly problematic. Apart from the concerns mentioned above, I also see a few others. No point going into them though, it's clear that the weight of opinion is against the design.

Sorry for asking, but what's wrong with thick flanges? Why not use a standard ANSI or MSS flange?? All PV codes will let you use them without further calcs.

If you stay with this custom plate flange idea, then I'd encourage you to forget the stiffeners and design one properly per a PV code. IMHO if the flange requires stiffeners to resist bending, then clearly the flange is not thick enough. Also, I'd suggest a slip-on flange type design [I think this has already been mentioned]. This will put the hemi head INSIDE the custom flange making it a safer joint and will remove some significant stress raisers. [In your design, you are prone to losing the head entirely if a defect should cause the weld to fail under load.]

Finally, I say this almost flippantly because it goes without saying, but if you ever do build this beast per your design, don't test it pneumatically!

Cheers

Rob

ps. G'day to bananna bender Aybee.

 

[flamby]

Actually this piece of monument is required not as a regular service part, but just to pressure test the pipeline as NDT procedure. Hence fatigue is not an issue, nor is the life as its a one-time use.

I have no choice of bolt-spacing, bolt dia etc as these are already in place. To save the time, the ribbed thing was thought of but it looks like there are no takers :-(.

 

[robsalv]

Flamby - That's a decent sized pipe you have there - perhaps you should raise the question in a piping forum. Someone is bound to have experience in temp hydro enclosures for 60" piping.

The options for a temporary hydro enclosure are a lot wider and potentially more relaxed, however we had a spectactular hydrotest failure a few years back because of a shortcut on a temporary hydro enclosure, so we don't take it lightly anymore!

Taking a different tack altogether, you could use a flat enclosure - all it needs is to have its thickness calculated per a piping or PV code and holes drilled to suit the pipe flange. Or if you're willing to tolerate bending and possibly permanent deformation you could design the flat plate using the code formula but taking say 0.9 yield stress rather than the code allowable stress. This will come up with a lesser thickness so saving money... but if you're not careful you could end up with a wok!

Cheers

Rob

 

[codeeng]

I would actually question why you have flanges in the first instance. For the size and thickness shown on your sketch you're probably looking at a design pressure in the region 1600-1700 psi. This would require 60" Class 900# flanges and is outside the range of B16.47. For piping this thick you generally see all joints welded, in which case you might want to consider welding the head for testing.

 

[flamby]

codeeng,

I wanted to avoid welding to the existing flange as it will need grinding etc. at the end of exercise. But now I think, this is what I am left with as a last resort.

Thanks.

 

[jte]

flamby-

As you can see by the volume of responses, you have an interesting problem! Next time, though, tell us the whole story up front.

Welding the head to the flange (full pen, right!) is an option. Assuming that you don't have PWHT issues or some exotic metallurgy, the field machining of the flange gasket surface shouldn't be too horrible. We re-machine heat exchanger gasket surfaces in the field regularly.

Another potential solution would be to contact Expansion Seal Technologies at

http://www.expansionseal.com/html/hypg1.htm

Carber testing might also provide a solution but their website at

http://www.carbertesting.com/

appears to be down at the moment.

Good luck, and let us know what you wound up doing and how it worked.

jt

 

[codeeng]

If you weld the head to the flange you should plan on re-machining not grinding. What's on the other end of the pipe? Could the test be performed with the mating component(s)?

 

[flamby]

Thankyou all for adding your concerns here which guided me through this unusual beastly design.

The testing is over and the ribbed flanges were used as in my original link, however with slight changes. The test was satisfactory but the strange part was that I was lacking the clue to whether we are testing the system or the enclosure.

Both of them worked. The deflection on flanges were close, but higher than the computed ones, though there was no leakage (we used two O-rings). Perhaps some day I can hone this art and make a flamby's dish-end a better idea.

Regards.

Ciao.

 

[jte]

flamby-

Thanks for the update! Too often we don't hear whether or not an approach discussed here worked or failed.

This is certainly a unique approach. I think you should consider writing a paper about it to be presented/published at the ASME Pressure Vessels and Piping conference. Too late for the 2005 conference, but good timing to start thinking about one for the 2006 conference in Vancouver, Canada.

http://asmeconferences.org/pvp05/

jt